Chlorosilanes such as trichlorosilane and silicon tetrachloride can be prepared, for example, by reacting crude silicon with chlorine or hydrogen chloride. In the case of an industrial scale preparation, types of crude silicon whose silicon content is 85% by weight or more are usually employed as starting materials. Other constituents of crude silicon are mainly iron, aluminum, calcium and titanium, which are converted into their chlorides during the reaction with chlorine or hydrogen chloride. In addition to these metal chlorides, high-boiling compounds such as hexachlorodisiloxane and pentachlorodisiloxane are also produced.
Customarily, these residues are roughly separated from the chlorosilanes by distillation. Depending upon the distillation conditions, the distillation residue is present in the form of a suspension or as a solid and requires a separate processing step.
The distillation of the chlorosilanes is carried out as completely as possible, because any chlorosilanes which remain in the distillation residue can no longer be converted into useful products and therefore represent a loss in value. In those instances where the total residues to be disposed of are in the form of a suspension, this suspension has a typical composition of about 30 to 40% by weight aluminum chloride, 2 to 3% by weight iron chloride, 2 to 3% by weight titanium tetrachloride, 35 to 45% by weight hexa- and pentachlorodisiloxane, the remainder being silicon tetrachloride and about 1% by weight trichlorosilane. If such a suspension is pre-dried by evaporating the low-boiling components, for example continuously in a screw-conveyor dryer, a residue pre-dried in such a manner has a typical composition of about 80% by weight aluminum chloride and iron chloride, 16% by weight hexa- and pentachlorodisiloxane and titanium tetrachloride, and 4% by weight silicon tetrachloride.
German Patent No. 21 61 641 discloses the reaction of a chlorosilane distillation residue with water vapor accompanied by the formation of hydrogen chloride. However, an adequate reaction takes place only with a stoichiometric excess of water vapor, so that additional hydrochloric acid is produced from the unreacted water, which has to be disposed of.
In order to avoid the formation of additional hydrochloric acid, German Patent No. 36 42 285 proposes to perform the hydrolysis of the distillation residue in the presence of additional hydrogen chloride and to recirculate the unreacted water. According to German Offenlegungsschrift No. 37 42 614, an additional treatment with air can further reduce the residual chloride content in the hydrolysis residue.
All of these known processes are complicated and in some instances cause problems with liquid and gaseous waste products.
U.S. Pat. No. 4,690,810 discloses a process for the reaction of chlorosilanes with milk of lime, where the chlorosilanes contain up to 20% by weight of finely divided metals and between 0.1 and 5% by weight chlorides of aluminum, iron and titanium. A stream of chlorosilane is introduced by way of a dip tube into a milk of lime suspension which has a pH of at least 9 until the pH of the lime suspension drops to between 7 and 8. Since this process forms calcium chloride, the disposal problems are not solved but merely shifted to this reaction product.
German Patent No. 32 47 997 discloses a process for the purification of waste gas from the production of polycrystalline silicon in which chlorosilanes are washed out of a gas mixture containing hydrogen and hydrogen chloride with the aid of saturated hydrochloric acid. The hydrolysis products resulting from this treatment are obtained in the form of easily filterable solids. As is known, the chlorosilanes contained in the waste gas of such a plant are essentially dichlorosilane and trichlorosilane. The waste gas effluent contains about 1 g of silicon per m.sup.3.
This prior art reference provides no guidelines for technical procedures with respect to processing of residues of a chlorosilane distillation, because only pure gaseous chlorosilanes, essentially dichlorosilane and trichlorosilane to be precise, are reacted at very low concentrations at which neither chlorodisiloxanes nor titanium tetrachloride nor aluminum chloride and iron chloride, which make up the largest part of the distillation residue, are present. Moreover, the reference does not disclose any method for the treatment of the separated hydrolysis product. If a typical residue of the distillation of chlorosilanes, which were prepared by reacting crude silicon with chlorine or hydrogen chloride, is continuously reacted with hydrochloric acid pursuant to this reference and it is attempted to continuously filter off the solids, this attempt fails because a reaction mixture prepared in this way simply cannot be filtered.